1. Technical Field
The present invention relates to a liquid crystal display panel, and more particularly, to a liquid crystal display panel with unit pixels, each unit pixel having slits in a pixel electrode and photo alignment layers.
2. Discussion of the Related Art
A liquid crystal display module includes a liquid crystal display panel that displays images by changing an arrangement of liquid crystal molecules according to a received electric signal, a backlight assembly supplying light to the liquid crystal display panel, and cases in which the liquid crystal display panel and the backlight assembly are fixed to each other.
The liquid crystal display panel includes a plurality of pixels arranged in the form of a matrix in rows and columns. Each of the pixels includes a pair of upper and lower glass substrates facing each other, a pair of polarizers which are formed on outer surfaces of the upper and lower glass substrates, respectively, a liquid crystal layer sealed and interposed between the upper and lower glass substrates, a pixel electrode formed on an inner surface of the lower glass substrate, a common electrode formed on an inner surface of the upper glass substrate, and alignment layers formed on the common electrode and the pixel electrode. In a color liquid crystal display panel, color filters having the primary colors including, for example, a set of red, green and blue (R, G, B), or a set of red, green, blue, and at least one selected from cyan, yellow, magenta and white, are found on pixel electrodes of the pixels, or on common electrodes corresponding to the pixel electrodes. A set of adjacent pixels in, for example, a row, representing each of the primary colors (e.g., R, G, and B) may be referred to as one pixel and each of the adjacent pixels, representing one of the primary colors, may be called a subpixel. In this specification, a “unit pixel” or a “pixel” means a subpixel in the case of a color liquid crystal display panel, that is, one of the adjacent pixels representing one of the primary colors.
It is desired that images displayed on a liquid crystal display panel show the same display qualities even though they are viewed in several different viewing directions or viewing angles. To achieve the same display qualities, a method of expanding a viewing angle of the liquid crystal display panel, by making liquid crystal molecules have similar refractive index anisotropy characteristics (or similar anisotropic characteristics in terms of the refractive index) in several different viewing directions, has been used. The method has been applied to Vertical Alignment (VA) mode and Plane to Line Switching (PLS) mode LCDs. In the VA mode, the method is based on how well the liquid crystal molecules in a liquid crystal layer are vertically arranged with respect to the substrates, and in the PLS mode, the method is based on how well the liquid crystal molecules in the liquid crystal layer are horizontally arranged with respect to the substrates.
In order to obtain a VA-mode liquid crystal display panel having improved viewing angle, a photoalignment process has been developed. The photoalignment process permits maintenance of constant gradation levels in wide viewing angles by dividing a unit pixel into a plurality of domains and making liquid crystal molecules of each domain have different pretilt directions when no voltage is applied to liquid crystal molecules.
FIG. 1A is an enlarged schematic conceptual diagram illustrating first and second alignment layers of a conventional unit pixel and alignment vectors of liquid crystal molecules, formed thereon. A unit pixel 100 has first and second substrates 101 and 103 facing each other. The first substrate 101 has a first underlying substrate 401 and a first alignment layer 110 disposed on the first underlying substrate 401, and the second substrate 103 has a second underlying substrate 403 and a second alignment layer 120 disposed on the second underlying substrate 403.
Although not shown in FIG. 1A, it may be understood by those of ordinary skill in the art that polarizers perpendicularly crossing polarization or transmission axes 111 and 121 may be formed on outer surfaces of the first and second underlying substrates 401 and 403, respectively; that a thin film transistor and a transparent pixel electrode for applying a pixel voltage to the unit pixel may be formed between the first underlying substrate 401 and the first alignment layer 110; and that a color filter layer representing any one of the primary colors may be formed between the second underlying substrate 403 and the second alignment layer 120.
To describe alignment directions, the X-Y-Z three-dimensional coordinate system is illustrated in FIG. 1A. Principal axes or major axes of liquid crystal molecules on the first alignment layer 110 are perpendicular to the x-y plane, and have first and second alignment vectors 310 and 320. The first and second alignment vectors 310 and 320 represent alignments whose pretilts are oppositely directed along the x-axis. The alignments are pretilted on planes that are parallel to the x-z plane and perpendicular to the x-y plane. The first and second alignment vectors 310 and 320 are parallel to the first transmission axis 111 of a polarizer formed on the first substrate 101. Similarly, principal axes or major axes of liquid crystal molecules on the second alignment layer 120 are perpendicular to the x-y plane, and have third and fourth alignment vectors 330 and 340. The third and fourth alignment vectors 330 and 340 represent alignments whose pretilts are oppositely directed along the y-axis. The alignments are pretilted on planes that are parallel to the y-z plane and perpendicular to the x-y plane. The third and fourth alignment vectors 330 and 340 are parallel to the second transmission axis 121 of a polarizer formed on the second substrate 103.
Therefore, the first and second alignment vectors 310 and 320 are perpendicular to the third and fourth alignment vectors 330 and 340. Such alignment vectors may be made by sequentially arranging masks in predetermined regions on alignment layers and irradiating light, such as polarized ultraviolet (UV) light, to be tilted with respect to the masks. The processes for forming the alignment vectors are disclosed in United States Patent Application Publication No. 2010-0157223 published on Jun. 24, 2010, and United States Patent Application Publication No. 2010-0034989 published on Feb. 11, 2010, both of which are commonly assigned to the owner of this application, and incorporated by reference herein.
Liquid crystal molecules are pre-tilted when the principal axes of liquid crystal molecules adjacent to an alignment layer are tilted in a predetermined direction with respect to a direction perpendicular to the surface of the alignment layer. In order to effectuate pretilt, the liquid crystal molecules may be physically bonded with the material of the alignment layer. A pretilt angle refers to an angle at which the pretilt is made with respect to the direction perpendicular to the surface of the alignment layer. In other words, the pretilt angle refers to the degree of pretilt.
FIG. 1B is an enlarged conceptual diagram illustrating locations and directions of domain alignment vectors of a unit pixel, made by a sum of the alignment vectors of the first and second alignment layers in FIG. 1A. FIG. 1B is a plan view, (i.e., seen above the unit pixel 100 in FIG. 1A), and illustrates first to fourth domain alignment vectors 360, 370, 380, and 390. In FIG. 1B, the first domain alignment vector 360 is the sum of the first and fourth alignment vectors 310 and 340, and is formed in a first domain 210. The second domain alignment vector 370 is the sum of the first and third alignment vectors 310 and 330, and is formed in a second domain 220. The third domain alignment vector 380 is the sum of the second and third alignment vectors 320 and 330, and is formed in a third domain 230. The fourth domain alignment vector 390 is the sum of the second and fourth alignment vectors 320 and 340, and is formed in a fourth domain 240.
Therefore, the domain alignment vectors cross the x-axis or y-axis of the x-y plane at an angle of 45°. The first transmission axis 111 of a first polarizer (not shown) formed under the first underlying substrate 401 is parallel to the x-axis, while the second transmission axis 121 of a second polarizer (not shown) formed under the second underlying substrate 403 is parallel to the y-axis, which is perpendicular to the x-axis. Hence, the domain alignment vectors cross the transmission axes 111 and 121 at an angle of 45°.
FIGS. 1C and 1D are enlarged schematic conceptual diagrams illustrating arrangements of liquid crystal molecules when a low-gradation level voltage and a high-gradation level voltage are applied to a liquid crystal layer in a micro region I shown in the fourth domain 240 of FIG. 1B, respectively. Liquid crystal molecules 610 in FIGS. 1C and 1D are divided into liquid crystal molecules 611 and 613 adjacent to the first and second alignment layers 110 and 120, and liquid crystal molecules 612, 614, and 615 situated in the central portion of the liquid crystal layer.
Pretilts and pretilt angles of the principal axes of the liquid crystal molecules 611 and 613 adjacent the alignment layers are predetermined by the liquid crystal molecules physically bonding with molecules of the alignment layers using the alignment technologies, such as those described above. The pretilts and pretilt angles of the alignment layer-adjacent liquid crystal molecules 611 and 613 are determined by the alignment vectors 310, 320, 330, and 340 of the first and second alignment layers 110 and 120, regardless of the strength of a pixel voltage or an electric field applied to a space between a pixel electrode 500 and a common electrode 460 of a unit pixel. An arrangement of the central-portion liquid crystal molecules 612, 614, and 615 is affected by both the pretilt angles of the alignment layer-adjacent liquid crystal molecules 611 and 613 and the pixel voltage, which is applied to the space between the pixel electrode 500 and the common electrode 460.
If a low-gradation level pixel voltage is applied to the space between the pixel electrode 500 and the common electrode 460 of the unit pixel 100, the central-portion liquid crystal molecules 612, 614, and 615 are substantially perpendicular to the surfaces of the first and second alignment layers 110 and 120 as illustrated in FIG. 1C. Since the transmission axes 111 and 121 of the liquid crystal display panel are perpendicular to each other, the amount of light passing through the unit pixel from the backlight assembly is limited in a VA mode device having a normally black mode. Therefore, if the pixel voltage has a voltage level corresponding to the minimum gradation level, the amount of light passing through the unit pixel is the minimum amount of light passing through the unit pixel.
On the other hand, if a high-gradation level pixel voltage is applied to the space between the pixel electrode 500 and the common electrode 460 of the unit pixel 100, polar angles of principal axes of the central-portion liquid crystal molecules 612, 614, and 615 (i.e., polar angles referring to angles between the principal axes of the liquid crystal molecules and the z-axis), vary from polar angles of the principal axes of the liquid crystal molecules 611 and 613 adjacent to the first and second alignment layers 110 and 120 up to polar angles of the principal axes of the centermost liquid crystal molecules 612 and 615 having the maximum polar angles. Azimuth angles of the principal axes of the central-portion liquid crystal molecules 612, 614, and 615 (i.e., azimuth angles referring to angles between projection lines of the principal axes of the central-portion liquid crystal molecules on the x-y plane and the x-axis), vary from an azimuth angle of the principal axis of the liquid crystal molecule 611 adjacent to the first alignment layer 110 up to an azimuth angle of the principal axis of the liquid crystal molecule adjacent to the second alignment layer 120, as illustrated in FIG. 1D.
Therefore, if the highest-gradation level pixel voltage is applied to the space between the pixel electrode 500 and the common electrode 460, the polar angles of the centermost liquid crystal molecules 612 and 615 approximates about 90°, whereas the azimuth angle thereof crosses the transmission axes 111 and 121 of the liquid crystal display panel at an angles of about 45°. If such characteristics of liquid crystal molecules are considered with respect to the VA mode device, light from the backlight assembly is subject to linear polarization by passing along the first transmission axis 111 of the first polarizer; thereafter, the light is subject to elliptical or circular polarization by passing through the central-portion liquid crystal molecules 612, 614, and 615; finally, the light is subject to linear polarization by passing along the second transmission axis 121 of the second polarizer, making it possible for a sufficient amount of light to pass through the unit pixel. Therefore, if the highest-gradation level pixel voltage is applied to the unit pixel, the amount of light passing through the unit pixel from the backlight assembly is the maximum amount of light passing through the unit pixel.
Unlike those of the central-portion liquid crystal molecules 612, 614, and 615, polar angles and azimuth angles of the alignment layer-adjacent liquid crystal molecules 611 and 613 are determined by the alignment vectors 310, 320, 330, and 340 of the first and second alignment layers 110 and 120, without being changed by various gradation-level voltages or electric fields applied to the space between the pixel electrode 500 and the common electrode 460 as described above. Therefore, if a high-gradation level voltage is applied to the space between the pixel electrode 500 and the common electrode 460 as described with reference to FIG. 1D, the alignment layer-adjacent liquid crystal molecules 611 and 613 do not change the polarization of the light passing through them to elliptical polarization or circular polarization, thereby contributing to a reduction in the amount of light passing through the unit pixel. Accordingly, in order to increase the amount of light passing through the unit pixel, the polar angles and azimuth angles are changed by adjusting the pretilts of the alignment layer-adjacent liquid crystal molecules 611 and 613.